1. Field of the Invention
The present invention generally relates to laser scanning apparatus and, more particularly, is directed to a laser scanning apparatus for drawing pattern informations, e.g., image, mask pattern, etc. on a drawing medium such as film, photoresist used to make a printed circuit board and a semiconductor integrated circuit, light valve used in a big-screen display device and so on by laser beams.
2. Description of the Prior Art
Conventionally, when a picture is drawn on a drawing medium such as a light valve used in a big-screen display device and so on, as shown in FIG. 1, an image input system 33 is utilized, in which an optical fiber plate 32 is secured to a display screen of a cathode ray tube 31. The optical fiber plate 32 is formed of a bundle of a number of fiber optics, and the optical fiber plate 32 and a light valve 34 are bonded together by an optical bonding agent 35. When a color video projector such as a color television receiver or the like is formed by using the image input system 33, three cathode ray tubes for producing three colors of red, green and blue, for example, a red cathode ray tube 31R, a green cathode ray tube 31G and a blue cathode ray tube 31B are employed and a red light valve 34R, a green light valve 34G and a blue light valve 34B are respectively provided on the entire surfaces of the cathode ray tubes 31R, 31G and 31B, thus forming three image input systems 33R, 33G and 33B, respectively.
Then, a light from a white light source 41 forming a read light source is divided to provide red, green and blue lights to operate the light valves 34R, 34G and 34B of red, green and blue colors. More specifically, the light from the white light source 41 passes a ultra-violet filter 42 and an infrared filter 43 and then its first polarized component is reflected by a polarizing beam splitter film 44, whereby only a red light becomes incident on the red light valve 34R by a red transmissible dichroic mirror 45. A second polarized component is passed through the polarizing beam splitter film 44 and then becomes incident on a green reflecting dichroic mirror 46, whereby only a green light is made incident on the green light valve 34G. Of the second polarized components passed through the green reflecting dichroic mirror 46, only a blue light becomes incident on the blue light valve 34B by means of a blue transmissible dichroic mirror 47. Then, red, green and blue lights are projected on a screen 49 through a projecting lens 48 to thereby obtain a color image.
That is, images corresponding to incident images from the cathode ray tubes are formed by an electrooptic effect of liquid crystal layers within the light valves and resultant images are projected on the screen 49 as a color image in an enlarged scale (see The Journal of the Institute of Television Engineers of Japan, Vol. 38, No. 4(1984)).
Another example of a conventional image input system 52 will be described below. As shown in FIG. 3, in this image input system 52, a light valve 34 is disposed distant from the cathode ray tube 31 and a focusing lens system 51 is disposed between the cathode ray tube 31 and the light valve 34. While the focusing lens system 51 is represented by a single lens in FIG. 3, this focusing lens system 51 is formed, in actual practice, by several sets of combination lenses, each set being formed of several lenses. When the projector such as the color television receiver or the like is constructed by using this image input system 52, it is composed of three image input systems similarly to FIG. 2.
However, in order to obtain high resolution in the image input system 33 shown in FIG. 1, the number of fiber optics forming the optical fiber plate 32 must be increased considerably and a flatness of the optical fiber plate 32 itself must be increased, which unavoidably provides an expensive image input system. Further, considering the F number of the optical fiber plate 32, the F number must be reduced in order to increase sensitivity while the F number must be increased in order to increase resolution of image, which unavoidably provides a relation of antinomy. Therefore, it is very difficult to place the image input system in an optimum condition.
On the other hand, in the image input system 52 shown in FIG. 3, an expensive lens system is indispensable for obtaining image having no distortion and for increasing brightness. Further, since the focusing lens system 51 formed by the combination of several sets of lenses is disposed between the display screen of the cathode ray tube 31 and the light valve 34, a spacing between the cathode ray tube 31 and the light valve 34 is increased, which unavoidably makes the image input system large in size.
The image input systems 33 and 52 shown in FIGS. 1 and 3 have a common defect such that, in the process for projecting the image displayed on the cathode ray tube 31 onto the screen 49 in an enlarged scale, a loss of light is remarkably large, thus making it impossible to obtain bright image. Incidentally, a light utilizing ratio of the image input system 33 of FIG. 1 and the image input system 52 of FIG. 3 is both approximately 10% of the whole light, wherein both the image input systems 33 and 52 generate dark images. For this reason, it is necessary to increase brightness by using the cathode ray tube 31 of high brightness, which unavoidably consumes much power. Further, since the three light valves 34R, 34G and 34B are employed for a color display of the projector, the three cathode ray tubes 31 and so on must be provided in accordance with the three light valves 34, which increases a heavy and large-sized projector accordingly. Therefore, a small projector is not yet realized.